In order to probe the nature of oxytocin receptor (OTR)/Gα q / 1 1 protein coupling, we examined the effect of co-expression of OTR intracellular domains on oxytocin-stimulated phosphoinositide turnover in COSM6 cells overexpressing OTR and Gα q . Co-expression of Gα q enhanced the oxytocin response maximally at a pOTR/pGα q plasmid transfection ratio of 1:0.16. In cells co-expressing OTR and Gα q / 1 1 , oxytocin stimulated phosphoinositide turnover with an EC 5 0 of 48 nM. Co-transfection with plasmids expressing OTR intracellular domains inhibited oxytocin-stimulated phosphoinositide turnover by 23 ± 6% (1i), 37 ± 4% (2i), 55 ± 6% (3i), and 40 ± 6% (4i), respectively (P < 0.01). Expression of the 3i loop of the α 1 B -adrenergic receptor, which also couples to Gα q / 1 1 , inhibited phosphoinositide turnover by 35 ± 2% (P < 0.01), while expression of the 3i loop of the dopamine 1A receptor, which couples to Gα s , had no effect. While these data indicate a functional role for the OTR 3i loop, they also suggest that interactions with more than one intracellular domain probably mediate the coupling of OTR to the Gα q / 1 1 class of GTP-binding proteins.